Near Surface Mounted Carbon Fiber Reinforced Polymer Bars Research Articles

Shear behavior of composite stone beams reinforced with NSM longitudinal CFRP bars

Employing near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) bars in the tensile zone of bare stone beams can effectively improve their flexural performance. Meanwhile, shear failure could occur when excessive amount of flexural reinforcement is adopted. Seven composite stone beams were tested to study the shear behavior of composite stone beams reinforced with NSM CFRP bars. The test variables included the shear span to depth ratio and the reinforcement ratio. Digital Image Correlation (DIC) technology was used to capture the full-field strain of the beams. The results indicated that the composite stone beam exhibited ductile failure mode with visible deflection and multiple cracks formed in both constant bending zone and flexural-shear zone. With the decrease of shear span to depth ratio, the cracking load and shear capacity increased linearly but the deformation capacity decreased. For the same shear span to depth ratio of 2.5, the composite beam with a higher CFRP reinforcement ratio observed higher shear load-carrying capacity. DIC results demonstrated the shear force was mainly provided by the arch action before failure. A shear-resistance theoretical model based on Rankine's failure criteria, sectional mechanical analysis and experimental observations was proposed and verified with the current and previous test results.

Comparative study on the flexural strengthening of RC beams using EB CFRP sheets, NSM CFRP bars, P-SWRs, and their combinations

The externally bonded (EB) carbon fiber reinforced polymer (CFRP), near-surface-mounted (NSM) CFRP and the prestressed steel wire ropes (P-SWRs) has been proven to be feasible and reliable flexural strengthening methods for concrete structures. However, debonding issues in EB CFRP sheets (or NSM CFRP bars) and space requirements for edge and spacing clearance of NSM CFRP bars (or P-SWRs) has limited the application of these three strengthening techniques. Therefore, the combinations of these methods need to be investigated and clarified. In this paper, experimental investigations on the flexural performances of seven concrete beams are presented, in which three of the beams were separately strengthened with EB CFRP sheets, NSM CFRP bars, and P-SWRs, respectively, and three of the beams were combinedly strengthened with two of the three techniques. Test results demonstrated that the flexural performances of beams strengthened with the combination with EB CFRP sheets, NSM CFRP bars, and P-SWRs are between the flexural performances of the beams strengthened with the corresponding individual strengthening techniques. It is found that the P-SWRs is favorable for combinations. In case the P-SWRs was combined with EB CFRP sheets or NSM CFRP bars for strengthening, the cracking load was increased to more than 230% and the maximum ultimate load-bearing capacity was increased to 150% when compared with the control beam. The debonding which occurred more or less in the beams strengthened with EB CFRP sheets or NSM CFRP bars or their combination was successfully eliminated by using the combination strengthening technique with P-SWRs. Also, the combination of P-SWRs with EB CFRP sheets or NSM CFRP bars is also favorable to improve the ductility of strengthened beams.

Enhancing flexural capacity of RC columns through near surface mounted SMA and CFRP bars

This study explores the flexural behavior of reinforced concrete (RC) columns strengthened with near surface mounted (NSM) shape memory alloy (SMA) bars or carbon fiber reinforced polymer (CFRP) bars. Seven RC column specimens were designed and fabricated to study the influence of different variables on the flexural response of the strengthened columns. These parameters include type of NSM reinforcement (SMA bars or CFRP bars), ratio of NSM reinforcement, and effect of CFRP jacketing. The columns were tested under cyclic lateral loading with constant axial force. The flexural behavior of each specimen was examined in terms of peak load, failure load, drift ratios, displacement ductility, stiffness degradation, energy dissipation, and seismic damage index. The experimental results indicate that strengthening of RC columns with NSM SMA or CFRP bars improves the flexural behavior of the columns through increasing the lateral load capacity, reducing the stiffness degradation and increasing the cumulative energy absorption up to failure. Further enhancement in the lateral response of RC columns was obtained by combining NSM bars and CFRP jacketing as the later provides an additional confinement to the critical sections of the test specimens.

Fracture Properties of Wide-notch Concrete Beams strengthened with Near-Surface Mounted CFRP Rebar

Carbon fiber reinforced polymer (CFRP) is a promising engineering material nowadays. This study carried out four-point-bending fracture experiment on wide-notch-concrete beams reinforced with near-surface mounted (NSM) CFRP rebar, aiming at exploring the effect of CFRP rebar on beam fracture characteristics. The test comprised four groups concrete beam specimens (three for each) with varying reinforcing methodologies. The first group specimens were common concrete beams strengthened by one near-surface mounted CFRP bar, while the second group consisted of common concrete beams strengthened by two near-surface mounted CFRP bars. On the issue of the third group, there were three wide-notch concrete beams strengthened with one near-surface mounted CFRP bar. Lastly, three wide-notch concrete beams strengthened with two near-surface mounted CFRP bars form the group four. The study proposed various equations about the rectangular concrete beams strengthened with near-surface mounted CFRP rebar under tension-shearing condition. For both open cracks and closed cracks, the equations concluded stress intensity factor as well as extending angle in mode I and II crack types. Four-point-bending fracture experiments for wide-notch-concrete beams and numerical simulation experiments were included in this research. This research exhibited expanding direction of wing type crack under effective shearing stress by the superposition model of crack tip stress intensity factor and stress intensity factor. Four-point-bending fracture tests for wide-notch-concrete beams and numerical simulation tests are finished in this research. The tests demonstrate characteristics of crack expanding direction, extending angle, vertical displacement, maximum normal stress, shear strain rate, distribution of plastic zones. Combining the theory and reality, the experimental results were in close agreement with the numerical research.

Experimental Investigation on Flexural Behavior of Granite Stone Slabs with Near Surface Mounted CFRP Bars and Screw-Thread Steels

An experimental study that investigates the behavior of stone slabs strengthened in fixure with near surface mounted (NSM) technique using screw-thread steels and carbon fiber-reinforced polymer (CFRP) bars is presented. A total of ten full-scale stone slabs were tested under a four-point bending loading to investigate the effect of groove dimension, reinforcement ratios, and reinforcement materials on the flexural performance of stone slabs. The test results included failure characteristics, yield and ultimate capacities, deflection of midspan, and cracking behavior of stone slabs. The test results indicate that with the increase of groove height and groove width, cracking load and middeflection decrease by 6.4%–14.18%; however, failure load and middeflection increase by 4.7%–41.2%. Cracking load, failure load, and failure displacement of stone slabs adopting NSM screw-thread steels increased by 10.9%, 167%, and 617%, respectively, under the maximum reinforcement ratios of 0.629% over the control slab without NSM bars. Meanwhile, with the increase of reinforcement ratios, the failure mode transforms from brittle failure to ductile failure. The calculation results of strength are in agreement with the experimental results. Finally, it can be concluded that NSM CFRP bars are more effective than NSM screw-thread steels to improve flexural capacity with the same reinforcement ratios.

Flexural strengthening of reinforced concrete beams with pre-stressed near surface mounted CFRP systems

This study investigated the effectiveness of strengthening reinforced concrete (RC) beams with pre-stressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) systems was investigated. Eight RC beams were tested under four-point loading tests. An un-strengthened control beam was used for reference. Four beams were strengthened with pre-tension NSM CFRP bars. The last three beams were strengthened with post-tension NSM CFRP bars. The test results showed that strengthening with a pre-stressed NSM beam enhanced the flexural behavior of the beam compared to that of the control beam. The post-tension NSM systems provided the best enhancement to beam performance for the concrete cracking, steel yielding, and ultimate loads. The test results of post-tension NSM beams were compared with the finite element analysis (FEA) model to validate the proposed model. The reinforcement length and pre-stressing force were used as parameters to analyze the effects of the strengthening performance in the proposed numerical model.

Flexural Behavior of Stone Slabs Reinforced with Prestressed NSM CFRP Bars

A technique for reinforcing stone slabs with prestressed near-surface mounted (NSM) carbon fiber–reinforced polymer (CFRP) bars is proposed and investigated in this research. Six stone slabs were tested under monotonic loading. Among these slabs, one served as a control slab and was not reinforced, and two were reinforced with one or two NSM CFRP bars, and each bar was prestressed to 30% of the bar’s ultimate strength. The remaining three were reinforced with three NSM CFRP bars that had been prestressed to 0, 15, or 30% of the bar’s ultimate strength. The test results indicate that reinforcement with either prestressed or nonprestressed NSM CFRP bars shifted the failure mode of the stone slabs from abrupt fracture to a more ductile flexural failure. A remarkable improvement in the flexural response was obtained when the stone slabs were reinforced with prestressed NSM CFRP bars. In addition to exhibiting a higher cracking load, the stone slabs that had been reinforced with prestressed NSM CFRP bars exhibited cracks with a smaller width, and the strength utilization of the CFRP reinforcement was much higher compared to that of the nonprestressed reinforced slabs. An analytical model was developed based on a sectional analysis method to predict the flexural response of stone slabs reinforced with prestressed NSM CFRP bars. The predicted values show good agreement with the experimental results.

Flexural behavior of stone beams reinforced with prestressed NSM CFRP bars

This paper is devoted to experimentally evaluate the efficiency of prestressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) bars for reinforcing stone beams to improve their flexural performance. A total of nine stone beams, classified into three test sets (Set A, Set B, and Set C) according to the beam span and the cross-sectional dimensions, were reinforced with prestressed NSM CFRP bars and tested under monotonic loading. In each test set, one bare stone beam was also tested for comparison. In Set C, the reinforced stone beams were strengthened with CFRP-strip wrapping in the shear span to increase the shear capacity. The test parameters included the amount of NSM CFRP reinforcements and the prestressing force. It was found from the test results that the use of prestressed NSM CFRP bars shifted the failure of stone beams from abrupt fracture to a more ductile mode and the load capacity was significantly increased. Failure of the reinforced stone beams in Set A and Set B occurred either by slip of the CFRP reinforcement or CFRP debonding or flexural–shear failure. The reinforced stone beams with CFRP-strip wrapping in Set C failed in the flexural–shear mode accompanied by rupture of the CFRP strips. The load–deflection responses of the reinforced stone beams exhibited a three-stage behavior which was similar to that of the reinforced concrete (RC) beams. The CFRP-strip wrapping effectively enhanced the shear capacity of the reinforced stone beams and avoided the abrupt failure when the first crack occurred in the shear span. For the stone beams reinforced with a same amount of NSM CFRP reinforcements, the increase in the prestressing force resulted in an increase in the load capacity accompanied by a reduction in the deformation capacity when the failure was governed by CFRP debonding.

Experimental and Parametric Investigation of Response of NSM CFRP-Strengthened RC Beams

The present investigation deals with carbon-fiber-reinforced polymer (CFRP) rebar manufacturing, experimental flexural and shear response of near-surface-mounted (NSM) CFRP-strengthened reinforced concrete (RC) beams, and detailed parametric study of NSM-strengthened RC beams using Abaqus. The CFRP bars were manufactured locally using the twisting technique. Parametric study was performed by considering various key parameters which affect the strengthening of the RC beams such as a CFRP bar’s orientation, geometry, intergroove separation, type of groove, cross-section area of bars, and material strength. The parametric study is based on the nonlinear finite-element modeling using Abaqus for examining the flexural and shear behavior. All beams were tested under a two-point loading system. The numerical results obtained using Abaqus are validated with the experimental results. The present study results show a significant increase in the strength of RC beams strengthened with NSM CFRP bars. Furthermore, based on this study, guidelines for realistic structural applications and optimization of the use of NSM CFRP bars are proposed.

Flexural strengthening of reinforced concrete beams using prestressed, near-surface-mounted CFRP bars

This study investigates the effectiveness of using prestressed near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) to strengthen reinforced concrete beams. Five large-scale reinforced concrete beams were tested: one control, one strengthened with nonprestressed NSM CFRP bars, and three strengthened with NSM CFRP bars prestressed to 20%, 40%, and 60% of the ultimate strength of the CFRP. The flexural behavior of the prestressed concrete beams was compared with the nonprestressed beam and the control. Results indicate that all prestressed concrete beams failed due to rupture of the CFRP bars without any premature or debonding failure. Prestressing the embedded NSM CFRP reinforcement enhanced the overall performance of the beams by increasing the loads at cracking, yielding, and ultimate. When compared with the control beam, prestressed CFRP enhanced overall performance by decreasing crack widths and deflections, as well as delaying the formation of new cracks. In general, increasing the prestressing levels of the NSM CFRP reinforcement improved the overall flexural behavior of the beams at service and ultimate conditions, while decreasing the ductility of the beams.

Partially bonded near-surface-mounted CFRP bars for strengthened concrete T-beams

A partially bonded strengthening approach for reinforced concrete (RC) beams utilizing near-surface-mounted (NSM) carbon fiber reinforced polymer (CFRP) bars was investigated with the specific objective of improving deformability. A total of six RC T-beams strengthened with NSM CFRP bars of various unbonded lengths were tested. Test results showed a decrease of the stiffness at the post-yield stage of the load–deflection response in the partially bonded beams. This is caused by the delayed increase of the FRP strain within the unbonded length. As a result the beam deformability was increased as the unbonded length increased at the same applied load. Internal slip of the FRP bar and gradual concrete failure were observed near the ultimate state, which caused a complicate nonlinear behavior of the beams. An analytical model is proposed to address the complete beam behavior including the effect of slip of FRP reinforcement and gradual concrete crushing. This model was developed based on the compatibility of deformation of the partially bonded system and was able to represent the ultimate behavior of the beams well.

Effective methods of using CFRP bars in shear strengthening of concrete girders

This paper presents the results of an experimental study that investigated the shear strength contribution of carbon fiber reinforced polymer (CFRP) bars attached with concrete beams using a near surface mounted (NSM) technique. In this research, four concrete beams were cast with regular steel reinforcement in flexure. The control beam had typical shear steel and the other three beams were strengthened in shear with CFRP bars. Strain gauges were attached with the shear reinforcement of all four beams at various shear critical locations. Strains during loading to failure of the beams were recorded using a data acquisition system. The performance of the NSM technique was found to be very effective with no occurrence of delamination, debonding or fracture of FRP. Effective strains in the NSM CFRP bars were determined through analyzing the collected strain data. A new formula to calculate the nominal shear strength provided by NSM CFRP bars has also been proposed.